1. Field of the Invention
This invention relates to mobile telephony and, more particularly, to modulation of text telephone data for transmission via a mobile telephone medium.
2. Description of the Related Art
Text Telephone (referred to as TTY in North America) equipment has been used in the fixed telephone network for many years to transmit text and speech through the conventional telephone network. In such systems, a conventional telephone is connected to a terminal and the user enters the characters via a keyboard. Some types of TTY systems the telephone handset may be acoustically coupled to the terminal, while in other types of TTY systems the telephone may be directly connected. The terminal encodes the characters into tones (using a mapping code) which are then transmitted from the terminal through the normal voice path of the conventional telephone. A TTY terminal, in conjunction with a telephone on the receiver side, may decode the tones and display the characters on the terminal display.
In the United States, some cellular telephony systems support TTY services using a Baudot Code to map the characters using two tones: 1400 Hz and 1800 Hz. However, conventional digital cellular systems may not provide satisfactory character error rates for text transmitted in the speech channel using the traditional TTY modulation techniques developed for the fixed network. Thus, a new standard has been created to provide this cellular TTY service. The new standard is commonly referred to as Cellular Text Telephone Modem (CTM).
Situations may arise during a mobile call such as during a cell handover, for example, when the normal traffic channel may be preempted and replaced by the transmission of control information using an associated control channel. One example of a control channel used in Global System for Mobile Communications (GSM) systems is a fast associated control channel (FACCH). In a conventional communication apparatus, the loss of normal speech data during preempting may be inconsequential as speech decoders may inject predetermined voice patterns or white noise into the receive path. However, this same loss of data during a text telephone transmission may result in an unacceptable character error rate at the receiver.